So, my Dianne I got from the store she initially had eyes that were...well...off.They seemed cloudy. The pupil is not as black as it should be, and the rest of both her eyes is a milky white/grey color, similar to that of a betta going blind.

I am curious, what causes this?

Also, what should I be looking out for, with her, any precautions, and is there a chance she will actually go blind?

It's very hard to see in the pictures what I mean, but looking at any of my fish their eyes are blue, black, or sometimes the same color as their body, or even a maroon color. Not...this.

She tends to focus more on something, and doesn't notice me as fast as the other gals. Some of her food often goes missed when it is sinking, compared to say...LaLa or Misteltoe.

Alright lol. She...was the one in a cup/bowl for 2 weeks. Misteltoe was in a tank, and LaLa was in with ich-infested tetras (and she didnt get ich)... So it is very possible about the water being bad for her. I do have her in a 1 gallon quarantine, but I constantly clean my tanks/bowls according to their size. I was wanting to have everyone together as a sorority, but if she does end up blind I won't do that to her.
I've never heard of the lack of vitamin A? But I will mention she's a wholesaler's betta, and from the same store I've gotten the blues with the same disease (different times), mal-nutritioned bettas, rotted bettas, columnaris bettas... (which for me it's my job :) )

haha I made it my job.
I rescue betta fish, usually getting them from "I'm gonna flush" owners and "ehh it's going to die anyways get rid of it for me!" pet stores. Truthfully, I could so easily have gotten this store put out of business with a health inspection, but I didn't because I respect the owner, NOT the employee. I've got like...three 10s, three 20s (one long), one 40, one 80... plus numerous under 5 gallons!!!

most of the "near death" bettas like Blue (who did pass away to the columnaris and mystery disease combined!) are free :p I ask the store owner, and he usually doesn't care. he won't sell sick fish anyways - no one wants them! So I take them xD Then about 5 dollars send them out to new homes.